The detail in a color image represented by a video signal is visible because a relatively abrupt change of signal level occurs in one or more of its color components. This change of level is identified as an edge. A detail signal is generated whose level is a function of the rate of signal level change of the color components representing this edge. This detail (image enhancement) signal, when applied to the video signal from which it was generated, increases the abruptness or steepness of detail edges which enhances the visual sharpness of the video image. Image enhancement is generally applied to all areas of the image and is appropriate since it is a function of detail within the image. The enhancement process is described in numerous patents including U.S. Pat. Nos. 5,103,299 4,855,829, 5,696,852, and 4,994,915.
The detail signal in video cameras and scanners is adjustable. The greater the amplitude of the detail signal, the sharper the image. However, as the level of the detail signal is increased, a level is reached above which positive and negative spikes are generated that outline elements within the image and give the image a hard edged look. The detail level adjustment is a compromise between increased image sharpness, and image degradation from overly hard edges.
In image compositing, the subject is photographed against a colored backing, which is later replaced in the compositing process by the intended background. The bright color of the backing (typically blue or green) at the edge of the subject, often induces a large and inappropriate edge enhancement since the background to be inserted is not likely to be the bright color of the backing. This inappropriate image enhancement at the subjects edge causes a visible and objectionable hard edge outlining the subject in the composite image.
An objectionable outlining effect, seen as dark edges and white edges outlining everything in a scene, is eliminated by clipping or limiting the spikes generated by excessive enhancement. At any abrupt change of signal level, a step exists from a given level to a higher or lower level for one or more of the component colors. These two signal levels at the step establish lower level and upper level bounds. The spikes resulting from excessive enhancement extend below the lower level bound and above the upper level bound. It is the extension of the spikes below and above the unenhanced video signal that is clipped. These clips prevent the enhanced video signal from exceeding the upper level, or from going lower than the lower level. By clipping the enhancement spikes from the video signal, the objectionable outline and hard edges are eliminated while improving image quality and sharpness.